X-ray examination apparatus with exposure control

ABSTRACT

An X-ray examination apparatus includes an X-ray source, an X-ray detector and an exposure control system. The exposure control system is arranged to control the X-ray source so as to perform a test exposure at a low X-ray dose and to perform an X-ray exposure at a higher X-ray dose. The X-ray detector applies a control signal resulting from the test exposure to the exposure control system and the X-ray source is adjusted on the basis of this control signal. The X-ray exposure produces an X-ray image and the X-ray detector supplies an image signal representing this X-ray image. The exposure control system is arranged to adjust the X-ray detector to a low spatial resolution during the test exposure and to a high spatial resolution during the X-ray exposure. The X-ray detector preferably includes a sensor matrix having sensor elements arranged in columns and rows. The spatial resolution is adjusted by deriving the control signal and the image signal from large and small groups of sensor elements, respectively.

PRIORITY

Applicants claim priority and continuation-in-part to U.S. patentapplication, Ser. No. 09/718,250 filed Nov. 22, 2000 now U.S. Pat. No.6,594,339.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an X-ray examination apparatus which includesan X-ray source, an X-ray detector, and an exposure control system forcontrolling the X-ray source and the X-ray detector, which exposurecontrol system is arranged to control the X-ray source so as to carryout a test exposure with a low X-ray dose while producing a controlsignal by the X-ray detector, and to control the X-ray source on thebasis of the control signal in order to carry out an X-ray exposure witha high X-ray dose and to acquire an X-ray image by the X-ray detectorduring this exposure.

2. Description of the Related Art

An X-ray examination apparatus known in the art is GermanOffenlegungsschrift DE 43 30 787.

An object to be examined during a radiological examination, for examplea patient to be examined, is exposed to an X-ray dose in order to formone or more X-ray images of the object. The X-ray dose required forindividual patients to be examined is customarily adjusted individually.The X-ray dose is adjusted by the exposure control system.

The known X-ray examination apparatus carries out the test exposurefirst with a comparatively low dose, thus forming a test image. Duringthe formation of this test image, the patient to be examined is brieflyexposed to X-rays with a predetermined intensity and energy. BecauseX-rays are applied only during a short period of time during theformation of the test image, the X-ray dose for forming the test imageremains comparatively small. The test image is read out from the X-raydetector and digitized in digital grey scale values with, for example, abit depth of 10 bits. The exposure control system of the known X-rayexamination apparatus derives the X-ray dose which is required to formthe X-ray image during the X-ray exposure from the distribution of thedigital grey scale values. The X-ray dose required for the formation ofthe X-ray image is large in comparison with the X-ray dose required toform the test image. In the known X-ray examination apparatus, thelarger X-ray dose is obtained by using a longer exposure time with thesame intensity and energy of the X-rays as used during the testexposure. The known X-ray detector is provided with a sensor matrixhaving a large number of sensor elements.

It is a drawback of the known X-ray examination apparatus that thereading out of the X-ray detector after the test exposure requires thesame amount of time as the reading out after the X-ray exposure wherebythe X-ray image is formed. Consequently, the adjustment of the X-raysource for the X-ray exposure requires a comparatively long period oftime.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an X-ray examinationapparatus enabling faster adjustment of the X-ray source.

This object is achieved by an X-ray examination apparatus according tothe invention which is characterized in that the exposure control systemis arranged to adjust the X-ray detector to a coarse spatial resolutionduring the execution of the test exposure, and to adjust the X-raydetector to a fine spatial resolution during the execution of the X-rayexposure.

As the X-ray detector is adjusted to a coarser resolution during thetest exposure, fewer signal values need be read out from the X-raydetector. Consequently, the time required to read out the X-ray detectoris shorter. Moreover, only a smaller number of signal values, forexample brightness values, need be processed in order to derive thecontrol signal therefrom; consequently, less time will also be requiredto form the control signal. Because the X-ray examination apparatusaccording to the invention requires only a short period of time for theadjustment of the X-ray examination apparatus on the basis of the testexposure, the X-ray image will become available sooner. Morespecifically, only a short period of time elapses between the testexposure and the X-ray exposure. Consequently, changes are less likelyto occur between the test exposure and the X-ray exposure, for exampledue to motion in or of the object to be examined. The X-ray examinationapparatus according to the invention is thus capable of quickly formingan X-ray image while ensuring that the X-ray image has a high diagnosticquality. This means that small details of low contrast are clearlyreproduced nevertheless in the X-ray image. The adjustment of the X-rayexamination apparatus concerns notably the adjustment of the energy andthe intensity of the X-rays emitted by the X-ray source.

These and other aspects of the invention will be described in detailwith reference to the following embodiments, limited only the scope ofthe claims appended hereto.

Preferably, the invention is used in an X-ray examination apparatusprovided with an X-ray detector having a sensor matrix with a largenumber of sensor elements. The individual sensor elements convertincident X-rays into electric charges. The sensor matrix also includesread-out lines via which the electric charges formed in the sensorelements can be read out or detected. During the test exposure, electriccharges are formed in the sensor elements. The separate signal levels ofthe control signal are composed of the electric charges in acomparatively small number of or in comparatively large groups of sensorelements. Such a large group contains a large number of sensor elements,i.e. more than one sensor element in any case. During the execution ofthe X-ray exposure, electric charges are formed again in the sensorelements. Any residual charges due to the test exposure are removed fromthe sensor elements. Such removal of electric charges is also called“electric reset” and is performed, for example by simultaneously readingout and draining the electric charges via all read-out lines. It is tobe noted that the electric reset is known per se from U.S. Pat. No.5,905,772 in which it is applied to counteract ghost images caused byelectric charges left behind in the sensor matrix. After the X-rayexposure, the electric charges are read out again so as to form theimage signal. The separate signal levels of the image signal are derivedfrom a rather large number of or from comparatively small groups ofsensor elements. Such a small group of sensor elements contains only asmall number of sensor elements; for example, such a group contains onlya single sensor element. The small groups of sensor elements containfewer, preferably far fewer, sensor elements than the large groups ofsensor elements. It is to be noted, however, that the sensor elements ofthe large groups as well as those of the small groups preferably formpart of the whole assembly of sensor elements of the sensor matrix.Sensor elements may form part of a large as well as a small group.Furthermore, the sensor elements of the large as well as those of thesmall groups preferably have the same construction. For example, thesensor elements are constructed as photodiodes. The sensor matrix isprovided with, for example, 1000×1000 or even 4000×4000 sensor elements.For example, a small group contains only a single sensor element or twoneighboring sensor elements. For example, 4×4 or 32×32 large groups ofsensor elements are used for the test exposure. When electric chargesare combined from large groups of sensor elements, less time will berequired for the reading out of the sensor matrix. The spatialresolution of the X-ray detector is coarser as electric charges frommore sensor elements are combined during reading out. The spatialresolution represents the dimensions of the smallest detail that isfaithfully detected by the X-ray detector. It has been found that it isnot necessary for the control signal that very small details areaccurately detected. The advantage of the shorter period of time that isrequired for the reading out of the electric charges from the largegroups outweighs the loss of spatial resolution. Moreover, thesignal-to-noise ratio of the control signal is higher as larger groupsof sensor elements are used, i.e. as the electric charges of more sensorelements are combined during the reading out.

For the reading out of the X-ray image formed by the X-ray exposure,however, the fine spatial resolution of the X-ray detector is ofimportance, to ensure that small details in the X-ray image are taken upexactly in the signal levels of the image signal. This fine spatialresolution is achieved by deriving the separate signal levels of theimage signal from electric charges of respective small groups whichpreferably contain only one or two sensor elements.

Preferably, a test image is formed by the test exposure. A measuringfield is selected from such a test image. The measuring field relates toa part of the object to be examined, for example the patient, which isvery relevant for the adjustment of the X-ray source. The measuringfield is determined, for example, on the basis of the brightness valuesof the test image. The control signal is derived from the brightnessvalues of the measuring field. Because of the use of such a measuringfield it is avoided notably that the control of the X-ray source isinfluenced by a part of the test image which does not relate to the partof the object to be examined. Notably, X-rays which reach the X-raydetector practically without attenuation are prevented from influencingthe control of the X-ray source. The adverse effects of non-attenuatedX-rays on the adjustment of the X-ray source during the X-ray exposureare thus avoided. For example, a histogram analysis can be performed onthe brightness values of the test image, notably in order to preventparts of the test image which relate to substantially non-attenuatedX-rays from influencing the control signal. Underexposure of the imagingof relevant parts of the anatomy in the X-ray image is thus avoided. Itis to be noted, however, that from U.S. Pat. No. 5,608,775 it is knownto read out only a part of the columns of sensor elements during thetest exposure.

Preferably, the dose rate is derived from the control signal produced bythe test exposure. The dose rate represents the energy deposited on theX-ray detector by the X-rays per unit of time. The exposure controlsystem is preferably arranged to calculate the time integral of the doserate during the X-ray exposure. When the X-ray exposure is performedwith the same intensity and energy of the X-rays as used for the testexposure, the time integral of the dose rate can be calculated simply bymultiplying the dose rate found during the test exposure by the currentduration of the X-ray exposure. It is alternatively possible to use anintensity and/or energy of the X-rays during the X-ray exposure otherthan that used during the test exposure. In order to calculate the timeintegral of the dose rate it is necessary to take into account thedifference in dose rate during the X-ray exposure and during the testexposure. The dependency of the dose rate on the difference between thedose rate during the test exposure and the dose rate during the X-rayexposure can be calculated or determined by calibration measurements.The dependency of the dose rate on the difference between the dose rateduring the test exposure and the dose rate during the X-ray exposure canbe derived, for example, by performing two successive test exposures andmeasuring the dose rate at a different energy and/or intensity of theX-rays. The dose rate during the X-ray exposure can be calculated oraccurately estimated on the basis of these two measuring results for thedose rate, for example by interpolation or extrapolation.

The time integral of the dose rate, so the current X-ray dose, iscalculated, for example at regular intervals and compared with a presetX-ray dose. As soon as the current X-ray dose reaches or exceeds thepreset value, the X-ray exposure is terminated by deactivation of theX-ray source. The X-ray source is thus adjusted to deliver, therequired, preset X-ray dose. For example, the signal level of thecontrol signal represents the current X-ray dose. The exposure controlsystem compares the signal level of the control signal with a limitvalue. Whenever the current X-ray dose is calculated again, the signallevel of the control signal is adapted and the X-ray source isdeactivated as soon as the signal level exceeds the limit level. Thelimit level amounts to, for example 1 V. In X-ray examination apparatusit is common practice to deactivate the X-ray source when the exposurecontrol system supplies a control signal having a signal level exceedingthe limit value of 1 V. The invention can thus be simply implemented inan existing X-ray examination apparatus.

The exposure control system in another embodiment of the X-rayexamination apparatus according to the invention measures the mean X-rayintensity across a test field during the X-ray exposure. Such a testfield comprises a pre-selected group or groups of sensor elements; ithas been found that columns of sensor elements which are connected tothe same read-out line are suitable to act as a test field. This isbecause it has been found that the total charge in the sensor elementsof the test field can be detected within a short period of time. Suchdetection is possible notably by measurement across parasiticcapacitances of the sensor elements and switching elements via which thesensor elements are connected to the read-out line. The total charge canthus be detected within 100 μs; the duration of the X-ray exposure istypically from 2 to 10 ms or even 300 ms, so that the total charge canbe detected twenty, one hundred or even thousands of times during theX-ray exposure. The mean intensity across the test field is thus updatedduring the X-ray exposure so that the current X-ray dose is known duringthe X-ray exposure. Individual read-out lines are connected torespective read-out amplifiers. It has been found that, due to parasiticsignal transfer from the sensor elements connected (for example, percolumn) to the same read-out line, the output of the relevant read-outamplifier always presents a signal level which corresponds to the totalcharge in the sensor elements connected to the relevant read-out line.The total amount of the electric charges in the sensor elementsconnected to individual read-out lines can be measured at the output ofthe read-out amplifiers, irrespective of the control of the switchingelements whereby the sensor elements are connected to the read-outlines. The sum of the electric charges in the sensor elements connectedto the respective read-out line corresponds to the mean X-ray intensityincident across the sensor elements connected to respective read-outlines. Consequently, the mean X-ray intensity across the test field canbe continuously measured during the X-ray exposure. When sensor elementsare. column-wise connected to respective read-out lines, the test fieldcan be composed of one or more complete columns of sensor elements.

It is alternatively possible to use a plurality of read-out lines percolumn, each time groups of sensor elements of individual columns thenbeing connected to individual read-out lines. Such groups of sensorelements then comprise each time parts of columns. It is then possibleto compose the test field from a plurality of such groups of sensorelements. The shape and the size of the test fields can thus be selectedin dependence on the architecture of the connection of the sensorelements to the read-out lines. Preferably, a reasonable correspondenceexists between the test field and the measuring field; the mean X-rayintensity across the test field then yields, in conjunction with a smallcorrection for the difference between the test field and the measuringfield, the signal level for the control signal whereby the X-ray sourceis accurately controlled. In practice it often occurs that the testfield comprises one or more complete columns of sensor elements. In thatcase often large differences exist between the test field and themeasuring field and it is necessary to derive the control signal fromthe mean X-ray intensity across the test field and across the measuringfield, while taking into account the difference between the test fieldand the measuring field. A suitable procedure for taking into accountthe difference between the measuring field and the test field is thedetermination of the ratio of the mean X-ray intensity across themeasuring field to the mean X-ray intensity across the test field duringthe test exposure. This ratio is represented by a correction factorwhich amounts to the ratio of the mean X-ray intensity across themeasuring field during the test exposure to the mean X-ray intensityacross a test field during the test exposure. This ratio can be readilycalculated on the basis of the brightnesses in the parts of the testimage which correspond to the test field and the measuring field,respectively. The mean X-ray intensity across the measuring field canthen also be derived during the X-ray exposure by multiplying themeasured mean current X-ray intensity across the test field by thecorrection factor. Subsequently, time integration is applied to thecurrent mean X-ray intensity thus calculated in order to calculate thecurrent X-ray dose relating to the part of the patient to be examined.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiments described hereinafter and the accompanying drawing; therein:

FIG. 1 shows diagrammatically an X-ray examination apparatus accordingto the invention,

FIG. 2 shows diagrammatically a sensor matrix of the X-ray detector ofthe X-ray examination apparatus shown in FIG. 1, and

FIG. 3 is a graphic representation of the variation of various signalsillustrating the operation of the exposure control system of the X-rayexamination apparatus according to the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 shows diagrammatically an X-ray examination apparatus accordingto the invention. The X-ray source 1 emits an X-ray beam 3 whichirradiates an object 2 to be examined, for example a patient to beradiologically examined. The X-ray image is formed on the X-ray detectoras a result of local differences in the X-ray absorption within thepatient 2. The X-ray detector in the present embodiment includes anX-ray sensitive solid state sensor matrix 20 with a large number ofsensor elements 5 in which electric charge carriers are released byincident X-rays. The electric charges in the sensor elements aredetected (read out) via read-out amplifiers 6 so as to be applied to amultiplexer (MX) 22. For example, the read-out amplifiers form columnsignals whose signal levels represent the electric charges in sensorelements in respective columns of the sensor matrix. The multiplexercombines the signal levels of the column signals so as to form a primaryimage signal (pIS) having signal levels representing the brightnessvalues of the X-ray image. The primary image signal is corrected forknown error sources by a post-processing unit (PP) 7. These correctionsconcern, for example faults introduced by defective sensor elements, bysensor elements having a deviating sensitivity, faults in the read-outlines or addressing lines of the sensor matrix and, for example,undesirable crosstalk and undesirable correlations occurring uponsimultaneous reading out of neighboring rows and columns of sensorelements 5. The post-processing unit delivers a corrected image signal(cIS), for example an electronic video signal, to a monitor 8 on whichthe information of the X-ray image is reproduced. The corrected imagesignal may also be applied to a buffer unit 9 in which the signal levelsof the corrected image signal are stored. Using the signal levels storedin the buffer unit 9, a hard copy of the X-ray image can be formed at alater stage or further image processing operations can be performedthereon.

The X-ray examination apparatus also includes an exposure control system10. The exposure control system is arranged to control the X-raydetector so as to produce a control signal (cS) with a coarse spatialresolution and to adjust the X-ray detector to a fine spatial resolutionin order to produce the primary image signal. The exposure controlsystem applies control signals (r-c1, c-c1) to a row driver (RD) 21 andthe multiplexer 22 in order to adjust the spatial resolution during thereading out of the sensor matrix. On the basis of the control signal(cS) the exposure control system derives an X-ray control signal (cX)which is applied to a high-voltage generator 11 of the X-ray source 1.It is alternatively possible to use the post-processing unit 7 to derivethe control signal (cS). In that case the multiplexer forms a mux signalin which signal levels of a plurality of column signals are summed.Subsequently, the post-processing unit 7 is used to sum signal levels ofthe mux signal from a plurality of rows of sensor elements in the sensormatrix, thus forming the control signal (cS). Furthermore, thepost-processing unit 7 is connected to the exposure control system asindicated by a dashed line in FIG. 1.

The X-ray control signal (cX) deactivates the high-voltage generator 11when the desired X-ray dose is reached. The X-ray control signal alsocontrols the intensity and the energy of the X-rays. The X-ray controlsignal is used notably to control the high-voltage (kV) across thecathode and the anode of the X-ray source and the anode current (mA) ofthe X-ray source. The exposure control system 10 derives the X-raycontrol signal (cX) from the current X-ray dose (DX) and/or from thedose rate r_(DX) as will be described in detail hereinafter withreference to FIG. 3.

FIG. 2 shows diagrammatically a sensor matrix of the X-ray detector ofthe X-ray examination apparatus shown in FIG. 1. For the sake ofsimplicity a sensor matrix comprising 4×4 sensor elements is shown byway of example. In practice use is made of sensor matrices comprisingmuch larger numbers of sensor elements, for example 1000×1000, 2000×3000or even 4000×4000 sensor elements. In the present embodiment, 2520×1920sensor elements are preferably used. Each of the individual sensorelements has an active surface area of approximately 100 μm×100 μm or200 μm×200 μm. The sensor elements in the matrix are connected in rowsto respective addressing lines 41. The sensor elements in the matrix areconnected in columns to respective read-out lines 40. Individual sensorelements are connected to the relevant addressing lines 41 by switchingelements 43, for example thin-film transistors. The individual thin-filmtransistors are connected to the relevant read-out line 40 by way oftheir drain contact whereas their source contact is connected to therelevant sensor element and their gate contact is connected to therelevant addressing line. The row driver 21 applies address signals tothe individual addressing lines. The address signals are applied to thegate contacts of the thin-film transistors 43 in the relevant rows inorder to turn on the relevant thin-film transistors, i.e. to close theswitching elements. The electric charges in the sensor elements are readout to the integrating read-out amplifiers 6 via the thin-filmtransistors and along the read-out lines. Thus, complete rows of thesensor matrix are read out essentially simultaneously. The integratingread-out amplifiers 6 derive the column signals in the form of electricvoltages representing the respective electric charges in the sensorelements in the relevant columns. The column signals are combined in themultiplexer 22 so as to form the primary image signal or the controlsignal, depending on the adjustment of the X-ray detector 4 by theexposure control system 10.

FIG. 3 is a graphic representation of the variation in time of varioussignals illustrating the operation of the exposure control system of theX-ray examination apparatus according to the invention.

In order to carry out the test exposure, a block-shaped pulse acting asthe X-ray control signal cX is applied to the high-voltage generatorduring a brief period of time T_(p). As a result, the X-ray source emitsan X-ray beam during the short period T_(p). The high voltage betweenthe cathode and the anode of the X-ray source and the anode current arepre-adjusted for the test exposure. The energy and intensity of theX-rays are thus adjusted during the test exposure. X-rays havingtraversed the patient during the test exposure are incident on the X-raydetector, so that the X-ray detector produces the control signal (cS).The X-ray intensity is I_(t) during the test exposure. In the exampleillustrated in FIG. 3, the signal level of the control signal (cS)represents the mean intensity over the pre-adjusted measuring field. Oneor more test images are formed by the X-ray detector during the testexposure. These test images are formed on the X-ray detector by theX-rays having traversed the patient to be examined during the testexposure. The exposure control calculates the mean intensity across themeasuring field on the basis of the test image (images). During theshort period T_(p), the signal level of the control signal increasesapproximately step-wise, the number of steps is equal to the number oftest images and the level reaches the value <m>, being the meanintensity across the measuring field. On the basis of the value <m> theexposure control system calculates an estimate of the dose rate D_(est)expected to occur during the X-ray exposure. This first estimate of thedose rate is indicated in the graph of the dose rate r_(DX). Theexposure control system calculates the actual X-ray dose DX from thecalculated dose rate r_(DX). This calculation takes into account thenon-linear dependency of the dose rate on the control signal. Notablynon-linearities occur due to adjusting phenomena of the X-ray sourcewhich occur for a brief period of time after the activation of the X-raysource during the test exposure. Due to the short duration of the testexposure, such adjusting phenomena are significant whereas they occur toa comparatively lesser extent during the longer X-ray exposure.Moreover, a difference, if any, between the settings of the high voltageand the anode current of the X-ray source for the test exposure and forthe X-ray exposure also induces a non-linear relationship between thecontrol signal and the estimated dose rate d_(EST). The non-linearrelationship between the signal level of the control signal and theestimated dose rate is measured, for example, during one or morecalibration exposures. This calculation of the estimated dose rate isindicated in the block “kV, mA”.

The exposure control system also calculates the mean X-ray intensityacross the test field from the test image. The test field concernsnotably all, or practically all, columns of the sensor matrix. Theexposure control system subsequently calculates the value of thecorrection factor${C = {\frac{\left\langle I_{i\quad j} \right\rangle_{m\quad f}}{.\left\langle I_{i\quad j} \right\rangle_{c\quad f}}.}},$wherein I_(ij) represents the brightness values in the pixel ij and <>_(mf), < >_(cf) are the mean values across the measuring field and thetest field, respectively. The calculation of the correction factor isindicated by way of the block “C”.

During the execution of the test exposure, the X-ray detector is readout, by way of the control signals (r-cl, c-cl) applied to the rowdriver 21 and the multiplexer 22, in such a manner that electric chargesof, for example, 4×4, 16×16, 32×16 or 32×32 groups of several sensorelements are combined so as to form respective signal levels of thecontrol signal (cS). Thus, the control signal represents the test imagewhich has a comparatively coarse spatial resolution. This is because thesmallest details faithfully reproduced in the test image have dimensionscorresponding to the dimensions of the groups of, for example, 4×4 or32×32 sensor elements. Moreover, during the test exposure, the exposurecontrol system 10 measures the mean X-ray intensity across the testfield of a large number of columns by measuring the signal level at theoutputs of the relevant read-out amplifiers 6 by the multiplexer 22during the test exposure. Measurements are performed on the read-outamplifiers over the parasitic capacitances in the relevant columns whilethe thin-film transistors are (still) open.

After the test exposure, the X-ray exposure is performed. The durationT_(exp) of the X-ray exposure is considerably longer (for example, 10ms), than the test exposure in order to ensure adequate diagnosticquality of the X-ray image. The X-ray control signal has a longer blockpulse of a length (in time) T_(exp) in order to activate thehigh-voltage generator and hence the X-ray source during the X-rayexposure. The adjustment of the X-ray source in respect of the intensityand the energy of the X-rays can be the same as during the testexposure, but this adjustment may also be changed for the X-rayexposure. At the beginning of the X-ray exposure, the X-ray intensityI_(x) increases from the value I_(t) to the value I_(exp). Subsequently,the X-ray intensity is stabilized at the value I_(exp) on the basis ofthe X-ray control signal.

The mean X-ray intensity across the test field is measured at regularintervals of, for example, 0.1 ms in the course of the X-ray exposure.The multiplexer 22 each time picks up the electric voltages on theoutput of the read-out amplifiers 6 and produces the control signal (cS)again. During this phase of the X-ray examination the signal level ofthe control signal (cS) initially increases; this corresponds to theincreasing X-ray intensity during the readjustment of the X-ray source.On the basis of the signal level of the control signal derived from thetest field and the correction factor, moreover, the current dose rater_(DX) is calculated at regular intervals and also, by integration ofthe current dose rate, the current X-ray dose DX. The value of thecurrent X-ray dose is represented by an electric dose voltage in theexposure control system. The exposure control system is also arranged tocompare the current X-ray dose with the preset X-ray dose. This iscarried out notably by determining the instant at which the electricdose voltage reaches, for example, the limit value of 1 V. In responsethereto the X-ray control signal (cX) deactivates the X-ray source andthe X-ray exposure is terminated exactly when the required X-ray dose isreached. Customarily used high-voltage power supplies for the X-raysource are adjusted in such a manner that they are deactivated on thebasis of an X-ray control signal of 1 V. Therefore, in order to carryout the invention the control of the high-voltage power supply need notbe modified additionally.

The exposure control system includes a suitably programmed computer or(micro)processor for executing its functions and calculations.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the spirit and scope of the invention. Thescope of the invention is indicated in the appended claims, and allchanges that come within the meaning and range of equivalents areintended to be embraced therein.

1. An X-ray examination apparatus, comprising: an expose control system;and an X-ray detector having an adjustable spatial resolution so as toderive a detector signal from incident X-rays, wherein said exposurecontrol system adjusts the spatial resolution of said X-ray detector toderive a control signal from a plurality of brightness values in ameasuring field.
 2. The X-ray examination apparatus of claim 1, whereinsaid X-ray detector is provided with a sensor matrix with a plurality ofsensor elements for converting incident X-rays into electric charges,and wherein individual signal levels of the detector signal are derivedfrom respective groups of sensor elements and a magnitude of therespective groups of sensor elements is adjustable.
 3. The X-rayexamination apparatus of claim 1, further comprising: an X-ray sourcecontrolled by said exposure control system based on the control signal.4. The X-ray examination apparatus of claim 1, wherein the measuringfield is selected by said exposure control system from a test image. 5.The X-ray examination apparatus of claim 4, wherein test image is formedby said exposure control system.
 6. The X-ray examination apparatus ofclaim 5, wherein said X-ray source carries out an exposure with a highX-ray dose.
 7. The X-ray examination apparatus of claim 6, wherein anX-ray images is acquired by said X-ray detector during the X-rayexposure.
 8. The X-ray examination apparatus of claim 7, wherein anumber of pixels of the test image is smaller than a number of pixels ofthe X-ray image.